            \begin{tikzpicture}
            %------------------------------------------------------------------------------------
            \node (a) at (0,0){(1)关键字（k）---var，integer;};
            \node (b) at ([xshift=-1.7em,yshift=-1em]a.south) {(2)标识符（i）---a，b;};
            \node (c) at ([xshift=0.1em,yshift=-1em]b.south) {(3)常\quad 数（c）---2，5;};
            \node (d) at ([xshift=0.8em,yshift=-1em]c.south) {(4)界\quad 符（p）---， ; : := +};
            \node (e) at ([xshift=-4em,yshift=1em]a.north) {单词类码};
            
            \begin{pgfonlayer}{background}
            \node [fill=yellow!10,fit=(a) (b) (c)(d)] {};
            \end{pgfonlayer}

            %------------------------------------------------------------------------------------
            \end{tikzpicture}